This invention relates generally to high-performance analog-to-digital (A/D) converters and, more particularly, to superconducting A/D's, employing superconducting Josephson junction devices. There is a requirement for high-performance A/D conversion equipment in both military and commercial applications. Two important measures of A/D converter performance are speed, i.e. the sampling rate in samples converted per second, and resolution, as measured by the ratio of the largest to the smallest increment of change that can be detected in an analog signal. Resolution is sometimes referred to as the dynamic range. Many applications require both high sampling rates and high resolution. Other important measures of A/D converter performance are sensitivity and linearity. Conventional techniques employing semiconductor circuitry have not been able to satisfy this requirement.
A/D conversion using Josephson junctions has been described in the technical literature. John P. Hurrell et al. described one such technique in a paper entitled "Analog-to-Digital Conversion with Unlatched SQUID's," published in the IEEE Transactions on Electron Devices, Vol. ED-27, No. 10, pp. 1887-96 (October 1980). SQUlD is an acronym for Superconducting Quantum Interference Device.
The theory of operation of SQUID's for use in A/D conversion is explained in detail in the Hurrell et al. paper, and only a simplified explanation will be provided here by way of background. Similarly, the theory of operation of Josephson junctions is now widely known, and has been the subject of discussion in many technical papers. For example, see B. D. Josephson, "Supercurrents through Barriers," Advan. Phys., Vol. 14, pp. 419-51 (1965), and other papers cited in the Hurrell et al. paper.
A Josephson junction has a current-voltage characteristic that includes a region in which the current increases rapidly from zero, with no corresponding increase in voltage across the device. A SQUID is a circuit including one or more Josephson junctions and one or more inductive loads. A single-junction SQUID includes a Josephson junction connected across an inductance. If a current is injected into one end of the inductance and the other end is grounded, the resulting characteristics provide the basis for A/D conversion, as explained in detail in the Hurrell et al. paper. Similar characteristics pertain to the two-junction SQUID, and in fact to n-junction SQUIDs.
The most pertinent property of the SQUID, from the standpoint of A/D conversion, is to be found in the relationship between the magnetic flux in the SQUID and the value of the injected current. This flux-current relationship is a periodic function and, depending on the circuit parameters chosen, a multi-valued function. The most significant aspect of the relationship is that the flux changes by a small quantum whenever the current increases by a small and precisely repeatable increment. This quantum of flux gives rise to a small but measurable voltage pulse across one junction. When the current is decreased, a flux quantum of opposite polarity is produced for each precise decrement of current, and a corresponding voltage pulse of opposite polarity is produced across a junction.
This property of the SQUID forms the basis for the A/D converter described in the Hurrell et al. paper. A signal to be converted from analog to digital form is introduced into the SQUID as a varying current in the inductor. Each time the current increases or decreases by a predetermined increment, a measurable voltage pulse is generated across the junction. In this manner, the SQUID functions as a quantizer. The resultant pulses are then detected and counted in one or more counters. The principal advantages of the arrangement are: (1) its near perfect linearity, (2) its sensitivity, and (3) its large dynamic range. The current increment, which determines the sensitivity, can be extremely small. The flux quantum is only 2.02.times.10.sup.-15 weber and the current increment is given by this value divided by the load inductance (measured in henries).
A double-junction SQUID comprises two Josephson junctions and a center-tapped inductance. The end terminals of the inductance are connected to a terminal of each of the junctions, and the other terminals of the junctions are connected together to ground. A control current is injected across the inductance, and a gate current is injected at the center tap of the inductance.
The double-junction SQUID circuit is bistable if the currents are appropriately chosen and controlled. Basically, in each of its two stable states the circuit has a circulating current component that flows through both of the junctions and the inductance. The direction of the circulating current component determines which state the circuit is in. When the gate current is raised momentarily above a threshold level, one of the junctions generates a voltage pulse and the direction of the circulating current reverses Subsequent pulses applied to the gate current toggle or reverse the state of the SQUID. Multiple circuits working on this principle can be connected in a cascade arrangement to operate as a binary scaler, counting the number of pulses from the quantizer.
A/D converters employing SQUID's have been disclosed in a number of patents, including U.S. Pat. Nos. 3,949,395 to Klein, 4,646,060 to Phillips et al. and 4,672,359 to Silver. Inherently, all high-performance A/D converters are limited by their resolution and their sampling rate. A measure of comparison that is sometimes used is the product of the resolution and the sampling rate. The sampling rate limitation of a SQUID quantizer is due to an inherent limit in the pulse rate that a SQUID can generate. The single flux quantum (SFQ) response time of the quantizer depends principally on the junction parameters. For example, the SFQ response time to achieve a 9-bit resolution at a sampling rate of 1.5 gigasamples per second (GSps), is approximately 1 picosecond (ps). The response time limits either the sampling rate or the resolution. If greater resolution is required, the quantizer will have to count more pulses per second, but this pulse rate is limited by the SFQ response time. Once the response time limit is reached, the resolution can be increased only at the expense of the sampling rate, and vice versa.
The present invention provides a solution to this apparently impenetrable barrier to increased sampling rate and resolution of a SQUID A/D converter.
SUMMARY OF THE INVENTION
The present invention resides in a superconducting analog-to-digital converter in which multiple quantizers are employed to provide interpolation between adjacent quantization levels of a single quantizer. The multiple quantizers provide a selected number of low-order bits of significance in the digital output, and can, be employed to increase the resolution of the converter, or, to increase the sampling rate, without the need for an inherently faster switching response time in the quantizers.
Briefly, and in general terms, the A/D converter of the invention comprises a plurality of quantizers in the form of superconducting quantum interference devices (SQUID's), configured to generate voltage pulses at periodically spaced quantization levels of an analog signal current applied to the quantizers; means for applying an identical analog signal current to each of the plurality of quantizers; means for applying a different bias current to each of the plurality of quantizers, such that the quantization levels are offset from each other in a uniform manner to provide for interpolation between adjacent quantization levels of a single quantizer; and means for monitoring the voltage pulses output by the quantizers to determine the digital equivalents of changes in the analog signal current. The differently biased quantizers provide a set of quantization levels interleaved between two adjacent quantization levels of a single quantizer.
The means for monitoring the voltage pulses typically includes at least one binary counter. Two such counters are presently preferred, to ensure that no counts are lost while one counter is being unloaded.
More specifically, the number of quantizers is 2.sup.n, and the means for monitoring the voltage pulses includes an N-bit bidirectional binary counter for counting pulses from one of the quantizers, and decoding means coupled to receive inputs from the quantizers, to generate n-bit output signals indicative of the analog signal current level in relation to the biased quantization levels of the plurality of quantizers. The converter provides an (N+n)-bit digital output equivalent to the accumulated change in the analog input signal. The conceptually simplest approach for decoding this converter output is to couple each of the quantizers to a binary counter and then to digitally add the binary signals from all of the counters.
In the illustrative embodiment of the invention, each of the quantizers has an associated divide-by-two circuit to provide an indication of a binary state of the quantizer. The converter also includes clocking means to control reading of binary signals from the divide-by-two circuits.
The decoding means of the illustrative embodiment includes a plurality of exclusive OR gates coupled to receive as inputs the states of the divide-by-two circuits associated with the quantizers, and to provide 2.sup.n outputs that uniquely identify which of 2.sup.n states are represented by the binary quantization states of the quantizers. The decoding means further includes logic means for converting the 2.sup.n output of the exclusive OR gates to an n-bit binary output signal.
The key to providing an interleaver or vernier type of operation of the converter is the manner in which the multiple quantizers are biased to offset their quantization levels. If the vernier is to provide an n-bit output, 2.sup.n quantizers are needed. If the quantization current of a single quantizer is .DELTA.i, the bias currents in each of 2.sup.n quantizers are 0, .DELTA.i/2.sup.n, 2.DELTA.i/2.sup.n, and so forth up to (2.sup.n -1).DELTA.i/2.sup.n.
It will be appreciated from the foregoing that the present invention represents a significant advance in the field of superconducting A/D converters. In particular, the invention provides either increased resolution or increased sampling rate, or both, without requiring any improvement in the basic switching response time of the quantizers employed. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings.